Thermionic tube circuits



June 26, 1945. c. E. McCLELLAN 2,379,163

THERMIONIC TUBE CIRCUIT Filed Aug. 6, 1942 i S INVENTOR Cmu. ENE CLELLAN FIG- 3. BY I 6 nil-5 767W ATTOIIRNEY WITNESSES:

Patented June 26, 1945 srArss ATENT orncs Claims.

This invention relates to improvements in thermionic tube circuits, and particularly to circuits of the cathode-follower type.

A cathode-follower circuit comprises a tube having a load impedance connected to the oathode and common to both the input and output circuits of the tube. The circuit thus includes a negative feedback path between the anode-cathode circuit and the input of the tube. In a cathode-follower circuit, the potential of the cathode tends to follow the potential variations of the it is the chief object of this invention to provide an improved cathode-follower circuit.

In cathode-follower circuits constructed in accordance with the teachings of the prior art of which I am aware, and particularly in circuits used in transmitters for ultra high frequency communication systems, the driving or operating potential for the cathode-follower tube is usually supplied by a separate driving or operating tube which is connected to the input of the cathodeiollower tube through a transmission line or lines. In such systems, the load is taken ofi the cathode follower across a cathode or load impedance, and the potential drop across this impedance is impressed between the control electrode and cathode of the cathode-follower tube in such a direction as to counteract the potential supplied by the driving or operating tube. This condition is objectionable, since the operating tube must supply additional potential to exceed the potential required for the operation of the cathode-follower tube by an amount approximately equal to the load potential.

Accordingly, it is another object to provide a cathode-follower circuit of the general type described wherein this objection is' eliminated.

Another object is to provide a cathode-follower circuit wherein the potential required to operate the cathode-follower tube is substantially independent of the load potential of the tube.

In accordance with my invention, the potential impressed in the control circuit of the cathodefollower tube by reason of the load impedance is counteracted by a potential of the cathode-follower tube.

These and other objects are effected by my inventlon as will be apparent from the following description and claims taken in connection with the accompanying drawing, forming a part of this application, in which:

Fig. 1 is a. schematic illustration of one typical cathode-follower circuit constructed in accordance with the prior art;

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Fig. 2 is a schematic illustration of one embodiment of the circuit provided in accordance with the invention; and

Fig. 3 is a schematic illustration of a transmission line arrangement that may be used in the practice of the invention.

Referring to the drawing, Fig. 1 illustrates how the cathode followers heretofore used have been connected to the source of driving or operating potential. This circuit includes a driving or operating tube it for supplying the potential to operate a cathode-follower tube H, and means for connecting the output of the operating tube to the cathode-follower tube.

The operating tube i8 is illustrated, by way of example, as of the triode type having an anode it, a. cathode it, and a grid H. The tube is energized through an impedance 9 from a suitable source of B potential. The cathode i8 is connected to ground or the negative terminal of the B potential source through a small impedance 8. The grid H is connected to ground or the negative terminal of the source through an imce l9. The impedance 8 thus provided proper bias between the grid I7 and the cathode it. The cathode It may be energized by a heater, the connection of which is not shown.

The cathode-follower tube H is also illustrated, by way of example, as of the triode type having an anode 20, a cathode M, and a grid 23. The tube H is also energized from a suitable source of B potential. The cathode 2| is connected to ground through an output or load impedance 22,

and the control electrode or grid 23 is connected to the anode E5 of the driving tube I ll, across a grid leak 26, through a conductor 25 and a coupling condenser 28. The cathode of this tube may also be energized by a heater, the connection of which is not shown.

The output or load is taken 01? the cathodefollower tube II by conductors 2! and 28, which are connected, respectively, to the upper and lower ends of the load impedance 22 through coupling condensers 29. The potential drop across the loadimpedance 22 appears as an inverse or negative feed-back potential in the circuit of the control electrode 23. This condition is objectionable since to operate the tube H, the tube l0 must supply sufllcient positive potential to cause the tube H to operate in the desired region of its characteristicand sufllcient additional potential I to overcome this inverse or feed-back potential.

to cause it to operate in the desired portion of its characteristic and that the output potential of the cathode-follower tube which is impressed across the load impedance 22 is 2000 volts. Under such circumstances, an inverse or feed-back potential of 2000 volts appears in the control circuit by reason of the potential drop across the load impedance 22 and the operating tube I. must furnish, in addition to the 200 volts, 2000 volts to counteract the inverse 'or feed-black potential, or, a total positive potential of 2200 volts.

The improved circuit provided by this invention, one embodiment of which is illustrated in Fig. 2, eliminates the objection pointed out above by providing a two-conductor transmission line arrangement between the driving or operating tube l and the cathode-follower tube II. The transmission line arrangement comprises two choke coils 3| and 32 wound in juxtaposed relation on the central leg 33 of a soft iron spool 34. The coils 3| and 32 are arranged so that the coupling between the two approaches unity. One end of the coil 3| is connected to the anode ll of the operating tube In through a suitable lead and coupling condenser 35, and the other end of this conductor is connected by a suitable lead to the control electrode 23 of the cathode-follower tube One end of the coil 32 is connected to the same point of reference potential as the cathode IQ of the operating tube In by a suitable lead and a coupling condenser 36, and the other end of this coil is connected by a suitable lead to the caniode lead of the cathode-follower tube II at a point above the upper end of the load impedance 22. An impedance element 30 is connected between the two conductors of the transmission line arrangement near the cathode-follower end to match the impedance of the transmission line.

In this improved circuit a fluctuating current flows in the network between the operating tube l0 and the cathode-follower tube under the driving potential impressed by the tube l0. The current flows from the anode H of the operating tube I0 through the coil 3| to the control electrode 23 of the cathode follower tube H and returns through the cathode 2| and the coil 32 to the point of reference potential, as indicated by the arrows in Fig. 2. Coils 3| and 32 are so wound that the magnetic effect of the flow of the last-mentioned current to the cathode-follower tube II is counteracted by the magnetic effect of the return of the last-mentioned current to the operating tube |0. Therefore, the driving potential impressed by the tube I0 is not affected by the inductive action of the coils 3| or 32, and the coils function as a transmission line for this potential.

Whenthe circuit is in operation, the drop across the load impedance 22 causes current to flow through the coil 32. The current flows in a circuit extending from the upper terminal of impedance 22, through coil 32, through capacitor 36, to ground. The current induces an electromotive force in the coil 3| which, by reason of the close coupling between coils 3| and 32, is approximately equal and opposite to the inverse feed-back potential impressed directly in the control circuit of tube H by load resistor 22. The inverse feed-back potential is thus substantially counteracted and the actual driving potential becomes substantially independent of the load potential.

Fig. 3 illustrates another transmission line ar-v rangement that may be inserted, at the points indicated by the broken lines x and Y in 1'18. 2, as a substitute for the transmission line arrangement illustrated in Fig. 2. The transmission line includes a first coiled conductor 3| and a second coiled conductor 32' arranged concentrically of the coil formed by the conductor 3|. However, it is understood that the two coils may be arranged in any manner as long as the coupling between the two approaches unity. A variable condenser 31 is connected across the ends of the conductor 32' for tuning the resonant circuit firmed by the conductor 32' and the condenser The operation of a circuit including the lastmentioned transmission line arrangement is similar to the operation of the circuit illustrated by Fig. 2. The current arising from the impressed driving potential flows through the conductors 3| and 32' in opposite directions, as indicated by the arrows in Fig. 3. The output potential of the cathode-follower tube |l causes current to fiow in the conductor 32' and sets up a counter electromotive force therein which is substantially equal to the potential derived from the load impedance 22. A corresponding electromotive force is set up in the conductor 3|, because of its close coupling with the conductor 32. The electromotive forces set up in the conductor 3| counteract the inverse or feed-back potential which is impressed in the control electrode circuit, by the potential drop across the load impedance 22.

While my invention in its more specific aspects applies to a cathode-follower circuit in which the tube is of the high vacuum, hot cathode type, it is also applicable in its broader aspects to circuits including gaseous tubes, such as mercury pool discharge devices.

From the foregoin description, taken in connection with the drawing, it is seen that this invention provides an improved thermionic tube circuit of the cathode-follower type, wherein the potential required to operate the cathode follower is reduced and made substantially independent of the output potential by balancing out the inverse or feed-back potential due to the potential drop across the load impedance of the cathode follower.

While I have shown my invention in several forms, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit thereof, and I desire, therefore, that only such limitations shall be placed thereupon as are specifically set forth in the appended claims.

What I claim is:

1. A tube circuit comprising a tube having a control electrode, an anode and a cathode, an impedance having one end connected to said cathode and having its other end connected through an output circuit to said anode and through an input circuit to said control electrode, a pair of conductors in mutually inductive relation, one energized in accordance with the potential drop through said impedance and the other serially connected in said input circuit.

2. A tube circuit comprising a tube having a control electrode, an anode and a cathode, an impedance having one end connected to said cathode and having its other end connected through an output circuit to said anode and through an input circuit to said control electrode, two coupled coils, one connected through a capacitor t0 shunt said impedance and the other-connected serially in aid input circuit.

3. A tube circuit comprising a tube having a cathode and a control electrode, a source of potential for operating said tube, an output load impedance connected in said cathode circuit in such manner that the potential drop across said impedance appears as an inverse potential in said control electrode circuit, a circuit connecting said source of operating potential to said tube, said circuit including a pair of conductors coiled on a common soft iron core, one of said coils being connected to said control electrode and the other being connected in said cathode circuit, said coils being so constructed and arranged that the current flowing through the same induces a potential which is introduced into said control electrode circuit, said induced potential being of opposite polarity to said inverse potential and of sufllcient magnitude to substantially counteract said inverse potential.

4. A tube circuit comprising a tube having a cathode and a control electrode, a source of potential for operating said tube, an output load impedance connected to said cathode in such manner that the potential drop across said impedance appears in said control electrode circuit as an inverse potential, a circuit between said source of operating potential and said tube including at least two closely-coupled coils and a variable condenser connected across the ends of one of thecoils, said coils being so constructed and arranged that the current flowing throughthe same induces a potential which is introduced into said control electrode circuit, said induced potential being of such polarity and magnitude as to substantially counteract said inverse potential.

5. A tube circuit comprising a first tube having a cathode and a control electrode, a second tube for furnishing operating potential for said first tube, said second tube having a cathode and an anode, an output load impedance connected to the cathode of said first tube in such manner that the potential drop across said impedance appears as an inverse potential in the control electrode circuit of said first tube, a circuit connecting said second tube to said first tube, said circuit including a first conductor connected between the cathode of said second tube and the control electrode of said first tube and a second conductor connected'between the cathode of said first tube and a point of reference potential, the arrangement of said conductors being such that the current flowing through said second conductor induces an electromotive force in said first conductor, said induced electromotive force being of such polarity and magnitude as to substantially counteract said inverse potential.

6. A tube circuit comprising, a tube having a control electrode and a cathode, an output load impedance connected at one end to said cathode and at the other end to said control electrode, and a transformer having a primary winding connected in a path shunting said impedance and a secondary winding serially connected in the.

circuit of said control electrode.

CYRlL E. McCIELLAN. 

